Dental furnaces with temperature control devices have been used in the industry for a long time. Thus, it is for example already provided in DE 1 160 777 to keep the temperature in the muffle furnace constant. A muffle furnace of this kind serves to fire a dental restoration part in a muffle that comprises a negative mold of the dental restoration part that is to be achieved. In the illustrated solution, a pre-heating chamber is provided below the actual firing chamber, and the heating power of the local firing chamber heating is controlled or driven via a continuous transformer. A “chopper bar” controller that may be regarded as a precursor of a PID controller, controls the heating power.
The solution described in the aforementioned patent specification is rather complicated or cumbersome if it is necessary to provide a quick cycle time for dental restoration parts and an ergonomically favorable workflow. This is due to the fact that a pre-heating zone is integrated there so that a cold muffle must typically be only heated there before it can be subjected to the firing cycle by insertion thereof into the press chamber.
However, it is particularly disadvantageous that in this solution the furnace head rests on fixed columns and remains stationary whereas the sensitive dental restoration part must be moved. This solution is indeed quite low-priced but it does not meet higher quality standards unless an extremely long cycle time is aimed for, during which the dental furnace completely cools down until the firing cycle is completed.
In case of modern dental furnaces, however, it is important to realize a shorter cycle time also when casting or pressing multi-unit bridges. Press furnaces in which a press stamp or plunger presses a green body into a channel in the muffle and the dental restoration part is produced in the negative mold as soon as the pressed article or blank that is also referred to as green body, liquefies, enable a high-quality production of a dental restoration part in a short cycle time.
The temperature control of firing furnaces and press furnaces for the dental technology has basically been employed unchanged for several decades. As a further example for the use of a thermocouple element in combination with a temperature controller DE 26 56 316 is to be mentioned.
Also DE 101 36 584 A1 illustrates a dental furnace comprising a muffle, the special feature in this case being the fact that the temperature sensor quasi penetrates into the muffle and is to detect the temperature thereof. For this purpose, however, a specific muffle form is necessary which is not compatible with commercially available muffles. Moreover, when realizing the particular recesses in the muffle, it has to be paid attention to the fact that the cavities for the dental restoration parts do not get too close to said recesses since at a corresponding high press power of the press stamp or plunger the muffle can form a crack at this location due to the weakening there.
A precise temperature control can thus be better achieved with the solution according to DE 10 2005 015 435 A1 in which a temperature sensor that is basically spaced apart from the muffle, measures the temperature thereof according to the type of a pyrometer for example. The temperature measurement is carried out at a position which is clearly spaced apart from the surface facing the heating elements so that the true muffle temperature is detected.